Fiberoptic telecommunications are continually subject to demands for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM). A DWDM system is capable of simultaneously transmitting many different and separate data streams on a single optical fiber. Each data stream represents a different channel on the optical fiber, where each channel exists at a different channel wavelength. The modulated output beam of a laser operating at the desired channel wavelength creates the data stream. Multiple lasers, each at a different wavelength, are used to create multiple data streams, whereafter the data streams are combined onto a single fiber for transmission in their respective channels.
The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers.
With the requirement for multiple tightly spaced channels, stable control over the laser source's output frequency is important to system effectiveness. The lasers used in DWDM systems typically have been based on distributed feedback (DFB) lasers operating with a reference wavelength tuning etalon in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Due to manufacturing as well as performance limitations, DFB lasers are used as single channel lasers, or as lasers limited to tuning among a small number of adjacent channels. As a result, DWDM applications require multiple different DFB lasers each at a different channel wavelength.
Continuously tunable external cavity lasers have been developed to overcome the limitations of DFB lasers. These lasers have a gain chip media and an end mirror that define an external cavity within which wavelength tuning occurs, e.g., by thermal tuning operation. Tuning is somewhat difficult to achieve given the complexity of the tuning element. The carriers used to retain the tuning elements are complex structures, designed to provide sufficient thermal isolation. Tuning operation may be effected by the carrier's thermal responsiveness, as well as its structure and orientation. As a result, i complex carrier shapes have been proposed, but even these shapes are still too complex to build.